1. Field of the Invention
The present invention relates generally to a gas turbine engine, and more specifically to an air-cooled turbine stator vane with endwall leading edge cooling.
2. Description of the Related Art Including Information Disclosed Under 37 CFR 1.97 and 1.98
In a gas turbine engine, a high temperature gas flow is passed through the turbine to produce mechanical work to drive the compressor and, in an industrial gas turbine engine, to also drive an electric generator and produce electrical energy. Passing a higher temperature gas flow into the turbine can increase the efficiency of the engine. However, the turbine inlet temperature is limited by the material properties of the first stage stator vanes and rotor blades as well as the amount of cooling that can be produced by passing cooling air through these airfoils (vanes and blades). Airfoil designers try to minimize the amount of cooling air used in the airfoils since the cooling air is typically bled off from the compressor and thus is not used to produce work and the energy used to compress the air is thus wasted.
A row of segmented guide vanes are located directly upstream of a row of rotor blades and function to redirect the hot gas flow into the rotor blades. FIG. 1 shows a prior art guide vane for a large industrial gas turbine engine. A bow wave driven hot gas flow ingestion phenomenon is created when the hot gas core flow entering the vane row where the leading edge of the vane forms a local blockage that creates a circumferential pressure variation at the intersection of the airfoil leading edge location. The leading edge of the turbine stator vane generates an upstream pressure variation that can lead to hot gas ingress into a front gap. If proper cooling or design measures are not undertaken to prevent this hot gas ingress, the hot gas ingress can lead to severe damage to the front edges of the vane endwall as well as the sealing material between adjacent vane segments such as honeycomb under the ID (inner diameter) endwall.
FIG. 1 shows a general schematic view of the bow wave effect ahead of the turbine vanes. The high pressure ahead of the vane leading edge is greater than the pressure inside of the cavity. This leads to causes a radial inward flow of the hot gas into the cavity. The ingested hot gas flows through the gap circumferentially inside of the cavity and towards the lower pressure zones, and finally outflow of the hot gas at locations where the cavity pressure is higher than the local hot gas flow pressure.
In general, the size of the bow wave is a strong function of the vane leading edge diameter and the distance of the vane leading edge to the endwall edge. Since the pressure variation in the tangential direction within the gap is sinusoidal, the amount of hot gas flow penetrating the axial gap increases linearly with the increasing gas width. Thus, it is important to reduce the axial gas width to a minimum allowable by the tolerance limits in order to reduce the hot gas ingress.
The high heat transfer coefficient and high gas temperature region caused by the above-described bow wave ingress hot gas flow associated with turbine vane endwall leading edge region can be alleviated by incorporating a new and effective direct vortex cooling with discrete film discharge slots of the present invention into the prior art endwall leading edge cooling design for the stator vanes.